Operating Systems

================ Start Lecture #24 ================

RAID (Redundant Array of Inexpensive Disks)

5.4.2: Disk Formatting


5.4.3: Disk Arm Scheduling Algorithms

There are three components to disk response time: seek, rotational latency, and transfer time. Disk arm scheduling is concerned with minimizing seek time by reordering the requests.

These algorithms are relevant only if there are several I/O requests pending. For many PCs this is not the case. For most commercial applications, I/O is crucial and there are often many requests pending.

  1. FCFS (First Come First Served): Simple but has long delays.

  2. Pick: Same as FCFS but pick up requests for cylinders that are passed on the way to the next FCFS request.

  3. SSTF or SSF (Shortest Seek (Time) First): Greedy algorithm. Can starve requests for outer cylinders and almost always favors middle requests.

  4. Scan (Look, Elevator): The method used by an old fashioned jukebox (remember “Happy Days”) and by elevators. The disk arm proceeds in one direction picking up all requests until there are no more requests in this direction at which point it goes back the other direction. This favors requests in the middle, but can't starve any requests.

  5. C-Scan (C-look, Circular Scan/Look): Similar to Scan but only service requests when moving in one direction. When going in the other direction, go directly to the furthest away request. This doesn't favor any spot on the disk. Indeed, it treats the cylinders as though they were a clock, i.e. after the highest numbered cylinder comes cylinder 0.

  6. N-step Scan: This is what the natural implementation of Scan gives.

Minimizing Rotational Latency

Use Scan based on sector numbers not cylinder number. For rotational latency Scan is the same as C-Scan. Why?
Ans: Because the disk only rotates in one direction.

Homework: 24, 25

5.4.4: Error Handling

Disks error rates have dropped in recent years. Moreover, bad block forwarding is normally done by the controller (or disk electronics) so this topic is no longer as important for OS.

5.5: Clocks

Also called timers.

5.5.1: Clock Hardware

5.5.2: Clock Software

  1. Time of day (TOD): Bump a counter each tick (clock interupt). If counter is only 32 bits must worry about overflow so keep two counters: low order and high order.

  2. Time quantum for RR: Decrement a counter at each tick. The quantum expires when counter is zero. Load this counter when the scheduler runs a process. This is presumably what you did for the (processor) scheduling lab.

  3. Accounting: At each tick, bump a counter in the process table entry for the currently running process.

  4. Alarm system call and system alarms:
  5. Profiling

Homework: 27